A laboratory model of a deep ocean vent has convinced Japanese scientists that life on Earth began at the bottom of the ocean more than three and a half billion years ago.

The team from the Nagaoka University of Technology showed that the special conditions found in a hydrothermal vent can allow simple molecules to join up in long chains - a crucial step in producing the more complex life-giving molecules such as RNA and DNA.

Hydrothermal vents are powered by the heat of submarine volcanoes. Water is drawn down into the rock, boils and then steams out again into the cold ocean.

As these parts of the oceans are lined with many vents, particular molecules can go through the vents many times. This repeated hot and cold cycling of water was the vital procedure.

"But to preserve the product, we need to transfer it quickly to an environment which is pretty cold. This is what happens at hydrothermal vents. Inside the vent, the water is very hot. But outside the vent, the water is very cold."

Professor Matsuno says living creatures are assembly plants for themselves, but he wishes to find the assembly plant which created the first living creatures.

The Nagaoka scientists produced a model of a vent in the lab to mimic the environment found on the ocean floor. They then added the amino acid glycine to the system. They saw it combine with itself through polymerisation, a process in which one unit is added to another step-by-step.

Diglycine - two glycine units joined by a peptide bond - was the first to emerge. After several minutes of cycling, a molecule containing six glycine units was produced.

Heat provided the energy to drive the reaction, but the cool periods in the water cycle ensured the newly-synthesised molecules did not immediately break apart.

The Japanese team now plans to make more sophisticated models to see if they can synthesise more complex molecules.

Alien life

Hydrothermal vents have been one of the great scientific discoveries of the past 30 years. The heat and the nutrient-rich waters found around the vents sustain an amazing array of life from bacteria to crabs, despite the total absence of light.

Not only do they provide one possible explanation for the chemical origins of life on Earth, they also raise the intriguing prospect that the same mechanisms may also have allowed life to start on other worlds - even in our Solar System.

Some scientists think Europa, Jupiter's icy moon, may have similar volcanic features deep beneath its surface which are capable of sustaining microbial lifeforms.